Generation of large postinjection-induced seismic events by backflow from dead-end faults and fractures

Induced seismicity is common during hydraulic stimulation in fractured crystalline rock. Fluid injection pressurizes preexisting fractures, triggering slip and seismicity. Often, the largest induced events occur after the end of injection, which complicates efforts to manage seismic risk. In this study, a three-dimensional discrete fracture network simulator that couples fluid flow with earthquake simulation was used to investigate a novel hypothesis for why large postinjection seismic events occur. Fractures that form dead-end pathways differentially pressurize during injection. After injection is stopped, fluid backflows through the well from the dead-end fractures into larger fractures, inducing additional seismicity and potentially causing events larger than occurred during injection. Our simulations indicate that flowing fluid back to the surface immediately after injection could mitigate this effect and reduce postinjection seismicity.